Single chamber, rate-responsive pacing is emerging as a new modality in cardiac pacing and in the near future, dual chamber rate-responsive pacing may be the optimal solution for most pacemaker patients. In this report we describe our short- and long-term clinical experience with two different rate-responsive pacemakers: the RS4, an asynchronous atrial sensing ventricular pacemaker, and the TX-pacemaker, which senses the evoked QT after a ventricular paced beat, as an indicator of metabolic demand. Both systems use a single ventricular lead. Nine patients received RS4 and 10 patients received TX units. All of these patients had AV block and good ventricular function except for three patients with sinus node disease in the TX group. Between 1 and 3 months after implantation, a 24-hour Holter monitoring was performed, during which two maximal symptom-limited treadmill exercise tests (Bruce protocol) were conducted in VVI (70 bpm) and rate-responsive modes, in a random fashion. The mean follow-up was 25 months in RS4 group and 10 months in TX group. Significant improvements in patient exercise tolerance were found in the rate-responsive mode (9.0 vs. 6.6 METs in VVI) with similar results in both groups (RS4 and TX) despite higher ventricular pacing rates in the TX group (121 bpm vs. 102 bpm in RS4). An autolimited rate-responsive pacemaker-mediated tachycardia, induced by retrograde ventriculo-atrial conduction, was observed in a patient with an RS4.(ABSTRACT TRUNCATED AT 250 WORDS)